The present invention relates to a dispersion of solvent-suspendible pressure sensitive adhesive particles and a pressure sensitive adhesive sheet utilizing the properties of the dispersion, and more particularly, to a dispersion of solvent-suspendible pressure sensitive adhesive particles which are hard to swell in solvents and have excellent adhesion and a pressure sensitive adhesive sheet produced by employing the dispersion, wherein when the pressure sensitive adhesive sheet of the present invention is removed from an adherend, the paper-tearing phenomenon that the surface materials of adherend is picked up by the adhesive sheet hardly occurs.
The present invention also relates to a dispersion of solvent-suspendible pressure sensitive adhesive particles prepared by dispersing pressure sensitive adhesive particles, which is hard to swell in solvents and has excellent adhesion, and binders filling the spaces between the particles, wherein the adhesive particles transfer phenomenon that the pressure sensitive adhesive are transfered to the adherend as well as the above-mentioned paper-tearing phenomenon hardly occur.
Recently, there have come on the market office supplies prepared by placing pressure sensitive adhesive particles on a sheet material such as paper, which can be repeatedly adhered and removed, that is, the adhesive face of the sheet is adhered to and removed from the adherend, and it is again adhered to and removed from the adherend.
The above mentioned pressure sensitive adhesive sheet utilizes the property of pressure sensitive adhesive particles having a particle size of several microns to a few hundreds microns. That is, when the adhesive face of the pressure sensitive adhesive sheet is adhered to an adherend, the adhesive contact is made on innumerable points, since pressure sensitive adhesive particles are placed on the sheet like cupules of octopus. When the sheet is removed from the adherend, the surface materials of the adherend is not picked up since the sheet is adhered to the adherend on points, each point the adhesive being very small in area. The pressure sensitive adhesive sheet can be again adhered to the adherend since pressure sensitive adhesive particles are still exposed retaining their original properties. Such a pressure sensitive adhesive sheet that can be repeatedly adhered and removed can be prepared by coating a dispersion of pressure sensitive particles on the surface thereof. In that case, it is preferable that pressure sensitive adhesive particles having an average particle size of several microns to several hundreds microns are densely placed on the surface of the paper.
Further, it is preferable that the shape of the pressure sensitive adhesive particle is truly spherical. If the particles are crushed like a Rugby ball, the particles are adhered to the sheet in a plane. Therefore, the paper-tearing phenomenon occurs because of strong adhesion.
Still further, it is preferable that the distribution range of particle size is as narrow as possible. When the distribution range is broad, the adhesion of the pressure sensitive adhesive sheet is poor and accordingly such sheet is not practical since only those particles having a large particle size contact the adherend.
This kind of pressure sensitive adhesive particles, which is well known hitherto, is prepared by the suspension polymerization of acrylic monomer in water using an oil soluble polymerization initiator. It is not problem in case that thus obtained aqueous suspension of pressure sensitive adhesive particles is coated on a non-water-absorbing substrate sheet, but when the aqueous suspension is directly coated on a paper, the paper curls owing to water and accordingly the commercial value of the pressure sensitive adhesive sheet remakably lowers. In general, therefore, as a solvent, an organic solvent such as toluene, ethyl acetate or 1,1,1-trichloroethane is employed instead of water. Moreover, the advantages that the drying time is short and the mechanical stability is excellent are obtained by employing the organic solvent.
However, in conventional pressure sensitive adhesive particles, the particles swell in the solvent more or less and tend to adhere to form a film even in case of using a poor solvent having little affinity. Therefore, such pressure sensitive adhesive particles have a drawback in readhesion since the separation between paper layers occurs because of the adhesion in a plane between the paper and the adherend.